Within the last year we have synthesized 110 peptides for use in twenty-six separate studies. These studies are run by sixteen investigators within the NIAID, and will be described in detail by those investigators. The majority of the peptides have been used to prepare anti-peptide antisera in rabbits. Anti-peptide antisera reactive with mouse MHC class I molecules have been used to detect the Q8 and Q10 gene products in various tissues and mouse strains. Other sera reactive with C-terminal regions of the H-2Kb molecule have been used to detect alternate RNA splicing patterns in the gene products of the H-2Kb and H-2Dd genes. Finally, sera and a monoclonal reactive with regions of the N domain of the H-2Kb molecule have been used to determine the extent of variability among various H-2 molecules. Anti-peptide sera reactive with MCF and xenotropic type C retroviruses have been used as specific typing reagents for these viruses. Anti-peptide sera reactive with a Hepatitus A virus (HAV) VPg like sequence was used to immunoprecipitate HAV RNA extracted from virions. Anti-peptide sera specific for the alpha and beta chains of the I-A molecule have also been prepared. In addition to using peptides to make antisera, peptides have also been used to map determinants recognized by antisera made against intact proteins. This approach has been used for monoclonals against the Plasmodium falciparum sporozoite surface antigens and antisera against the Hepatitus B virus. Finally, many peptides have been made from sequences contained in cytochrome C and used to define the T-cell epitopes on this molecule.